The GMC Yukon is considered a full-sized SUV, while the Explorer is a mid-sized SUV. Check the price of a Yukon with a Ford Expedition for a valid comparison.
Probably because a Yukon, with that GM engine, can be expected to run much longer than an inferior Ford product.
Give GM it's due: their products may rust, shake apart, disintegrate, and eventually be reclaimed by the earth, but that engine (assuming a reasonable level of maintainance) will start every time you put the key in the ignition. You simply cannot kill a GM engine, short of gross abuse or explosives.
You can't say that about a Ford, which requires a prayer in order to start, if it's over three years old.